The motivation for the development of novel micro-mechanical test frames is grounded in the recent development of micro-architected cellular materials. Novel advances in their manufacturing and optimal design urgently require the development of versatile and accurate experimental techniques for mechanical characterization at the unit cell level. Wide force and displacement ranges are generally necessary, while nN and nm resolutions are needed to capture small-scale phenomena. The ideal micromechanical test frame should be capable of measuring forces with resolutions in the 1-100 nN range with potentially large displacements (˜1 mm), allow optical (or SEM) access to the test coupon with potential for strain mapping (via Digital Image Correlation), be readily reconfigurable and adaptable to microstructures of a variety of shapes and sizes.
On-chip MEMS test frames have already been demonstrated. Although excellent for alignment purposes and resolutions, they lack the displacement range and versatility discussed above. A hybrid micro-test frame (comprising an off-chip actuator and a MEMS sensor) with the desired displacement range and resolution was recently introduced, but the compliant sensor limited the achievable force range. A limited number of fully integrated nanoindenter/SEM combinations exist today; but such devices are unique, highly customized, extremely expensive, and often limited in the maximum achievable displacement and/or force range.
The dependence of the resonant frequencies of structures on internal stresses had found applications in vibrating cylinder pressure transducers as early as the mid-1960s. A decade later, separation of the sensor element and the pressure chamber was shown to improve the resolution, resulting in one of the first demonstrations of axially loaded resonant load cells. This approach was subsequently applied to micro accelerometers and precision scales. The use of resonant force sensors for material characterization was first implemented at the macro scale. More recent developments in silicon micromachining techniques and brilliant yet simple design solutions for actuation and detection mechanisms led to micro-machined resonant force sensors, at first designed for accelerometer applications. The DETF structure is later proven to be a feasible design for a number of other micro sensor applications.
The governing mechanics of DETF sensors is well documented, as is their most recent application to accelerometers and gyroscopes.